1. Field of the Invention
The invention relates to a method for identifying floating cells, and in particular to a method for quickly identifying floating cells by a bit-line coupling pattern (BLCP).
2. Description of Related Art
There has been rapid growth in the demand for larger and faster memories for personal computers, workstations, communication systems, and graphics subsystems. These systems use memories to write, store, and retrieve large amounts of data. The data represent information or program instructions that are coded in combinations of the binary digits (bits) "1" and "0," and can be used by the microprocessor for further manipulation. Basically, there are two kinds of memories. One is nonvolatile memory (NVM), such as magnetic, optical media as well as some specially designed semiconductor ROM. The information last written in those memories is retained permanently, even after the power supply is removed. The other is volatile memory (VM), such as static random access memory (SRAM) and dynamic random access memory (DRAM). The volatile memories lose the stored information when the power is turned off.
In fact, the RAM which constitutes most semiconductor memory allows any part of the memory to be read or written as fast as any other part. The above-mentioned DRAM contains a great amount of DRAM cells, wherein each DRAM cell consists of one transistor and one capacitor. Currently, the high density and low cost of DRAMs have earned them a predominant role in computer main memories. During the past 35 years, the number of DRAM bits/chip has increased by a factor of four every three years and the cost/bit has declined by roughly the same factor.
However, during the manufacture of DRAM devices, floating problems of DRAM cells including open bit-line contacts and open DRAM cell contacts are encountered, resulting in the malfunction of DRAM devices. Generally, in the prior art, floating DRAM cells can be identified by use of physical failure analysis which includes focus ion beam (FIB) and secondary electronmicroscopy (SEM) approaches. When using the focus ion beam approach to identify floating DRAM cells, the addresses of suspicious floating DRAM cells must first be located, then the located DRAM cells are cut by FIB to analyze whether the located DRAM cells are floating cells or not. Another FIB approach involves removing upper layers of the DRAM device to expose the required DRAM cells for observation. IF the DRAM cells are bright, it means that there are no floating problems. Conversely, if the DRAM cells are dark due to accumulated positive charges which can not flow, these observed DRAM cells are floating cells. The secondary electronmicroscopy approach involves first fixing the DRAM devices and then cutting them in order to directly observe the cross-section of DRAM cells to identify whether or not there are floating DRAM cells. Nevertheless, the above-mentioned approaches for identifying floating DRAM cells have the disadvantages of low efficiency and high cost.